It is well known that a remote unit's location within a wireless communication system may be determined using a trilateration method. According to such a method, distances between the remote unit and multiple base stations are calculated based on a measurement of time delay of a signal traveling between the base station and the remote unit and then to each base station (round-trip delays). Such a prior-art method for calculating a remote unit's location is described in U.S. Pat. No. 5,508,708 "Method and Apparatus for Location Finding in a CDMA System" by Ghosh et al. and incorporated by reference herein. As described in Ghosh et al., when location of a remote unit is desired, the uplink signal transmitted from the remote unit to multiple base stations is analyzed to determine round-trip delay at each base station. From these delays, a distance is calculated from each base station to the remote unit, and the location of the remote unit is determined.
In a cellular environment, received signals undergo multipath scattering. In other words, a signal transmitted by a base station/remote unit undergo multiple reflections before being received at a receiver, and these reflections cause "echoes" (or copies) of the transmitted signal to be received by the receiver along with a non-reflected (or "prompt") component. These echoes are generally of different amplitudes and different time delays, and therefore cause received signals to actually consists of a multiplicity of signals (the actual signal and its echoes), each having a different amplitude, angle of arrival, and time delay. In a receiver incorporating the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95A (IS-95A), a RAKE receiver is utilized to lock onto each of the several multipath rays of a received signal with individual RAKE "fingers." The receiver combines fingers together to form a demodulated information stream that is later decoded to recover the transmitted data. Internal to the RAKE receiver a "searcher" (or search processor) will have knowledge of the time offsets and pilot power for each finger. It is generally regarded in trilaterization that determining the time delay of the prompt or at least the earliest ray will result in the most accurate location prediction.
According to IS-95A, the remote unit's transmission of an uplink signal is required to be within .+-.1 .mu.sec (microsecond) of the perceived first ray. Because of this, calculated propagation delay times utilized in remote unit location are assumed to be based off the remote unit transmitting within .+-.1 .mu.sec of the prompt ray. Unfortunately, there exist situations in which the remote unit is unable to resolve the prompt ray. For example, the prompt ray may be attenuated in amplitude, or may be close enough in time to a subsequent ray that the receiver cannot resolve the prompt ray. In general IS-95A RAKE receivers are unable to separate rays that are closer in time than one chip time (approximately 800 nsec). Because of this, a remote unit may fail to time align to the prompt ray, causing an error to be introduced in any determination of round-trip delay time which results in an inaccurate determination of remote unit location (via trilateration). Therefore, a need exists for a method and apparatus for determining if a remote unit fails to adjust its transmission based on reception of the prompt ray, and compensating a measured round-trip delay accordingly.